1. Field of the Invention
In conventional wireless communication networks, e.g., a cellular system, relays or relay stations are used to support the communication between a sender (source) and a receiver (destination) of the cellular system. The cellular system including the relay station may support the HARQ protocol. In such a scenario the source transmits information to the destination, e.g., via the relay station, and the code rate is matched to the channel (1st hop) between the source and the relay station. The destination may receive signals both directly from the source and via the relay station. The signals are combined and decoded at the destination. In case the destination fails to decode a received signal a negative acknowledgement message (NAK message—negative acknowledgement message) is issued. In response to the NAK message a retransmission is initiated in accordance with the HARQ protocol. In a known system, in case of such a negative acknowledgement not the source but the relay station is controlled to do the retransmission. This may be beneficial if the channel (2nd hop) between the relay station and the destination is better (e.g., has a better channel quality) than the direct channel between the source and the destination, which is quite likely. In a packet-based radio communication network the HARQ protocol is an important technique that allows an error-free transmission by requesting the retransmitting of any falsely decoded packet data. Known HARQ protocols/schemes are the Case Combining (CC) scheme and the Incremental Redundancy (IR) scheme.
In accordance with the information theory a successful decoding of a strong code like, e.g., a turbo-code or a LDPC code means that the mutual information (MI) (I(X;Y)) of a channel is larger than the code rate (Rc), wherein the behavior of I(X;Y) depends on SNR (SNR=Signal-to-Noise Ratio), i.e., I(X;Y)=f(SNR). Using the HARQ protocol allows for the retransmission of falsely decoded packets or signals thereby increasing I(X;Y) to exceed the code rate Rc.
FIG. 1 illustrates the dependence of the mutual information I(X,Y) on a distance between a source and a destination. FIG. 1A schematically illustrates a source S and a destination D. In case of an uplink, the source may be a user equipment UE, and the destination may be a base station eNB. The mutual information I(X,Y) defines the average information that leaves the source S at location X and reaches the destination D at location Y. Therefore, the mutual information I(X,Y) should be at a maximum. To ensure a successful decoding at the destination, the mutual information I(X,Y) needs to be greater than the code rate Rc. FIG. 1B depicts a graphical representation of the course of the mutual information I(X,Y) dependent on the distance between the source and the destination. As can be seen, the longer the distance between the source and the destination, the lower the mutual information I(X;Y), and at some point it falls below the threshold Rc set by the code rate. Below the threshold decoding at the destination will fail. The HARQ scheme is an approach that allows for a retransmission which will result in an increase of the mutual information I(X;Y) thereby exceeding Rc and thereby allowing longer distances between the source and the destination.
2. Description of the Related Art
U.S. Pat. No. 7,697,948 B2 describes an approach for retransmitting bits using the HARQ scheme but without using a relay station. US 2007/0190934 A1 describes a communication system using relay stations operating in accordance with a specific relay mode which is selected based on the mutual information MI. However, the HARQ protocol is not described. Y. Qi, R. Hoshyar and R Tafazolli describe in “On the Performance of HARQ with Hybrid Relaying Schemes,” in IEEE ICC 2009, a wireless communication system using relay stations and the HARQ protocol. The mutual information is used to switch between two preset relay functions. Thus, in conventional technology systems using relay stations and operating in accordance with the HARQ protocol the relay station is simply used for the retransmission under the assumption that the relay station is in the position to correctly decode the entire code word which can then be retransmitted. Resources needed for the retransmission are not considered.